1. Field of the Invention
The present invention relates to an angle potentiometer. More specifically, the present invention relates to an angle potentiometer provided for an angle detection unit and the like for outputting the detected angle as the duty ratio of a pulse signal.
2. Description of the Background Art
Conventionally, an angle detection unit is utilized to detect the angle of a work arm of a construction machine or the like. Since the construction machine is used out of doors, there is the problem of severe environmental changes such as temperature differences and voltage fluctuations. Thus, in the angular detection unit, a stable A/D conversion is performed that is unaffected by the temperature changes and power-supply voltage fluctuations so as to output the angle detection signal as a digital signal.
The A/D conversion portion of a conventional angle detection unit compares the voltage of the output from an angle detection sensor with the voltage of a triangular wave generated in the analog circuit including a resistance and a capacitor, varies the threshold value shaping the triangular wave, and varies the duty ratio of the logic high or "H" level to the logic low or "L" level in the output pulse.
With this method, since an analog triangular wave is generated by a resistance and a capacitor, the amplitude and the frequency of the triangular wave are caused to change due to the variations in the resistance value of the resistance and in the capacitance value of the capacitor owing to temperature changes. Consequently, the frequency of the output pulse and the duty ratio are varied. Particularly, the variation of the duty ratio has particularly proven to be a problem.
In addition, when the angle detection unit is used in an adverse environment, i. e. where water or mud is splashed over the housing of the angle detection unit, the water or mud enters the housing, often causing the output terminal and the high voltage line to become short-circuited. In such cases, a high voltage is applied to the output transistor, which leads to a possible malfunction of the output transistor or the like. As a consequence, conventionally such measures are taken as utilizing a special seal to block the water or mud from entering into the electronic circuitry of the sensor even in such an adverse environment, employing an output semiconductor device having a good voltage withstanding characteristic, and connecting a high-speed fuse in series to the output. The special seal, however, has become expensive. Moreover, the signal characteristic of the output can be degraded, and sometimes the fuse does not respond in time. As a result, such disadvantages are noted as a higher cost of the sensor unit, the output specification that is less than satisfactory, and shortened lifetime of the electronic components of the internal circuit.
Thus, the main object of the present invention is to provide an angle potentiometer which decreases the variation of the duty ratio due to the variation of the power-supply voltage and which is capable of converting a stable analog signal into a duty ratio of a pulse signal.
Another object of the present invention is to provide an angle potentiometer that prevents the lifetime of the electronic components of the internal circuit from becoming short, by disconnecting the output at a high speed when an excessive voltage is added to the output signal.